Changes the markdown headers used in the changelog to ## from ###
Changes the markdown headers used in the changelog to ## from ###

This change makes it so that all of the official Stripe client libraries
have more similar markdown formatting in their respective changelogs.

Signed-off-by: zach wick <be63f901512811b6173577ab76dbe50e8720cb0b@stripe.com>

stripe/stripe-android,stripe/stripe-android,stripe/stripe-android,stripe/stripe-android,stripe/stripe-android